Promotions are approved by Congress - read the text of an officer’s commission. It’s at the pleasure of the Commander in Chief.
Yes... However, the difference between an officer that is selected and ‘frocked’ and one that is ‘permanent’ is the Congressional approval. The pay comes after Congress approves.
I was frocked and able to wear my LT (O-3) uniform about 3 months before congress approved it and I got paid. The higher up the longer the approval between frocking and approval/pay.
The process is: selection (from the service and executive branch)/frocking/approval by Congress. If Congress denies the frocking can be reverted. (I’ve only heard of one case of this during my time in the 90s - related to a DUI or drugs I forget)